1. Field of the Invention
The present invention relates to a redundant gateway system including multiple gateway devices provided between a single telephone network like a TDM network and multiple packet networks, like IP networks, which are simultaneously and concurrently operated.
2. Description of Related Art
Gateway devices are used, for example, to connect a telephone network and a packet network to each other. The gateway devices are often designed to form a duplex or more, redundant configuration to suppress effects on a user when one of the gateway devices breaks down. Usually, these redundant gateway devices operate with one device set as an active system and another device set as a standby system. Needless to say, the active and standby systems are switched therebetween when a failure occurs in the active system. Additionally, the active and standby systems periodically switch therebetween in preparation for a silence failure where it is difficult to pinpoint a failure. Japanese Patent Application Publication No. 2005-57461, for example, discloses a method for avoiding interruption of communication at the time of this system switching.
However, in the disclosed technique described above, the standby system takes over from the active system, only part of RTP/RTCP session information and connectivity information, which are transmitted to a packet network, excluding a time stamp (TS) value and a sequence number (SN) value. Accordingly, the time stamp and sequence number values are discontinuous in RTP packets received by a receiver on the packet network side, and therefore a conflict with a telephone communication, which means instantaneous discontinuity of the telephone communication, cannot be avoided. On the other hand, during processing of voice on voice packets received from the packet network, with subsequent output of voice packets to a telephone network as voice data, the active system and the standby system cannot be synchronized. As a result, conflict between the telephone communication and duplication of voice are caused by the system switching.
This conflict particularly arises where multiple gateway devices of redundant configuration are respectively connected to multiple packet networks redundantly configured to form multiple systems. In this case, deficits, fluctuations, and delays of the packets that are concurrently transmitted from the multiple packet networks differ among the systems. Accordingly, in the system switching between multiple gateway systems, the system switching must be implemented without conflicts. That is, timing of voice data output from the multiple gateway systems must be adjusted while completely eliminating a drop of data. However, the conventional art does not implement the system switching without such conflicts.